A riot police officer during the distubances in Tottenham, north London. Photograph: Lewis Whyld/PA

Resting against the restored red brick of Tottenham's Edwardian town hall in north London is the marble memorial dedicated to one of Tottenham's late matriarchs, Cynthia Jarrett. The flowers that accompany the memorial may have browned with age but the memory of Jarrett's death is regularly revisited.

Since August comparisons have been frequently drawn between the deaths of Jarrett and Mark Duggan as preludes to Tottenham's respective riots.

However, although Duggan's death brought many outside Tottenham police station, the earliest reports on 6 August told that the trigger of the violence was not his death, but the beating of a 16-year-old girl of African descent by police during the peaceful stage of the demonstration.

Over three months have passed and the 16-year-old is yet to come forward, raising the questions, who is she, where is she – and does she exist?

Of the number who recounted the Tottenham riots from first memory, none could identify her, and despite their conviction that the story was true, only one said they saw it directly – a grandmother, who was also present during the Broadwater Farm riots.

The grandmother said: "I was angry and I was cursing, I remember the young girl was standing by the policeman's line. Where I was facing, I saw the back of her, so I saw the policeman lift his fist and punch her right in her mouth, as he did that I ran over to her and her mouth was bleeding, and from when that fist hit the girl, the riots started. That's how the riots started, when the police officer punched the 16-year-old girl in the face."

The Tottenham grandmother also confirmed herself as the voice heard on the unclear YouTube video that alleged to have captured the attack, and which has been broadcast all over the world and to many confirmed the rumour.

However, both the YouTube video and the grandmother's account conflict with other known facts – for example, the fact that the first police car was set on fire at 8.30pm, summertime, and therefore in daylight. This moment is said to have marked the start of the clashes. The YouTube video of the alleged attack was clearly taken during nightfall after the clashes would have begun. The testimony of the grandmother was also undermined by those who said they were with her that night.

Further doubts about the event were confirmed by Enid Ledgister, of the Haringey Community and Police Consultative Group, which monitors community and police relations in the area. Ledgister said: "We [HCPCG] have no knowledge of such an incident taking place."

Despite the absence of empirical evidence, the conviction among those who took part in the riots remains relevant.

That night, the story of the unidentified black teenager spread along Tottenham High Road faster than the fires. The video that seemed to confirm the rumour went viral a day later.

One Tottenham participant shared how the unconfirmed rumour spread on the day. "I was speaking to a random woman. I didn't know her at the time however we started a conversation and she went on to tell me that the police attacked a young girl. Now at the time I was not sure. I was completely unaware of all of this, however the following day I received an email link with footage of this young black girl being attacked by the police."

Even if the story is a fallacy its potential to be true in the aftermath of student protests and the case of Jody McIntyre, and the death of Mark Duggan, reflects how the expectation held by many of the police as an abuser rather than a servant of the public can be devastating.

The IPCC has investigated 460 deaths following police contact; these investigations have led – astonishingly – to zero convictions of officers. This has created a crisis in trust among sections of communities where a collective memory of police brutality against black men is remembered.

A 25-year-old local activist known as Kwabena Smiles told Reading the Riots researchers that, "echoes of the Cliff McDaniel attack, and deaths of Colin Roach, Cynthia Jarrett, is, for many locals, why there is such a deep distrust of the police. Thus why would this young lady come forward in hope of justice? Would you come forward if you were raised in an environment where many grandparents, parents and young people believe the police can get away with murder?"

The rumour also began to mutate with some hearing that she was a pregnant woman and others that the girl was 10 years old. One participant remembers his anger on hearing it respectively: "There's a video on YouTube where they're beating up this little 10-year-old girl … They was beating the little 10-year-old girl up. What did she do? What are they getting outta that? See. Things like that gets me more mad so I feel, yeah, there should be a fucking riot."

The changes to the story are similar to that in south London's Brixton riots of 1981 and the 2005 Lozells riots in Birmingham. During the first Brixton riots a young black man with a stab wound was held by the police before being released in an ambulance. Many involved heard the police were preventing an ambulance from being called, and some that it was the police that had stabbed him.

In Birmingham's race riots the parallels were even closer. Black youths took to the streets after it was broadcast that a black teenage girl had been raped by south Asian men. No evidence was ever found to confirm the rumour.

The prevalence of riot rumours that live past the disturbance shows how easily fiction can be considered fact in the presence of communal consent. Unless a girl comes forward or the 16-year-old is identified the attack will retain the status of myth, but as long as there is an expectation of police violence among sections of society, she will also continue to be remembered as a martyr.